GUIDELINES FOR THE GEOLOGIC EVALUATION

OF DEBRIS-FLOW HAZARDS ON ALLUVIAL

FANS IN UTAH

by Richard E. Giraud

Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use. The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product.

THE UTAH GEOLOGICAL SURVEY

Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah Geological Survey, makes nowarranty, expressed or implied, regarding its suitability for any particular use. The Utah Department of Natural Resources, Utah Geological Sur-vey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by usersof this product.

ABSTRACT ment and vegetation as they travel downchannel. When

flows reach an alluvial fan and lose channel confinement, The Utah Geological Survey (UGS) developed these they spread laterally and deposit the entrained sediment.guidelines to help geologists evaluate debris-flow hazards In addition to being debris-flow-deposition sites, alluvialon alluvial fans to ensure safe development. Debris-flow- fans are also favored sites for urban development; there-hazard evaluations are particularly important because fore, a debris-flow-hazard evaluation is necessary whenalluvial fans are the primary sites of debris-flow deposi- developing on alluvial fans. A debris-flow-hazard evalu-tion and are also favored sites for development. The pur- ation requires an understanding of the debris-flowpose of a debris-flow-hazard evaluation is to characterize processes that govern sediment supply, sediment bulking,the hazard and provide design parameters for risk reduc- flow volume, flow frequency, and deposition.tion. The UGS recommends critical facilities and struc- Evaluation of the debris-flow hazard follows thetures for human occupancy not be placed in active debris- premise that areas where debris flows have deposited sed-flow travel and deposition areas unless the risk is reducedto an acceptable level. These guidelines use the characteristics of alluvial-fan deposits as well as drainage-basin and feeder-channelsediment-supply conditions to evaluate debris-flow haz-ards. The hazard evaluation relies on the geomorpholo-gy, sedimentology, and stratigraphy of existing alluvial-fan deposits. Analysis of alluvial-fan deposits providesthe geologic basis for estimating frequency and potentialvolume of debris flows and describing debris-flow behav-ior. Drainage-basin and feeder-channel characteristicsdetermine potential debris-flow susceptibility and thevolume of stored channel sediment available for sedimentbulking in future flows. The debris-flow hazard depends on site location onan alluvial fan. Generally, sediment burial and impacthazards are much greater in proximal fan areas than inmedial and distal areas downfan. Hazard zones may alsobe outlined on the alluvial fan to understand potentialeffects of debris flows and determine appropriate risk-reduction measures. Geologic estimates of debris-flow-design parameters are necessary for the engineeringdesign of risk-reduction structures.

iment in the recent geologic past are likely sites for future (Blair and McPherson, 1994). Debris-flow and stream-debris-flow activity. Evaluation of the debris-flow hazard flow-flooding hazards may be managed differently inuses geomorphic, sedimentologic, and stratigraphic infor- terms of land-use planning and protective measures, butmation from existing debris-flow deposits and sediment- because debris-flows and stream-flow hazards are oftenvolume estimates from the feeder channel and drainage closely associated, concurrent evaluation of both debris-basin to estimate the hazard within the active deposition- flow and stream-flow components of alluvial-fan flood-al area of an alluvial fan. A complete debris-flow-hazard ing is often beneficial.assessment typically involves geologic, hydrologic, The purpose of a geologic evaluation is to determinehydraulic, and engineering evaluations. The nature of the whether or not a debris-flow hazard exists, describe theproposed development and the anticipated risk-reduction hazard, and if needed, provide geologic parameters nec-measures required typically determine the scope of the essary for hydrologists and engineers to design risk-hazard evaluation. reduction measures. The objective is to determine active Large-volume debris flows are low-frequency events, depositional areas, frequency and magnitude (volume) ofand the time between large flows is typically a period of previous flows, and likely impacts of future sedimenta-deceptive tranquility. Debris flows pose a hazard very tion events. Dynamic analysis of debris flows usingdifferent from other types of landslides and floods due to hydrologic, hydraulic, and other engineering methods totheir rapid movement and destructive power. Debris design site-specific risk-reduction measures is not ad-flows can occur with little warning. Fifteen people have dressed by these guidelines.been killed by debris flows in Utah. Thirteen of these These guidelines will assist engineering geologists invictims were killed in two different events at night as fast- evaluating debris-flow hazards in Utah, engineers inmoving debris flows allowed little chance of escape. In designing risk-reduction measures, and land-use plannersaddition to threatening lives, debris flows can damage and technical reviewers in reviewing debris-flow-hazardbuildings and infrastructure by sediment burial, erosion, reports. They are modeled after the Utah Geological Sur-direct impact, and associated water flooding. The 1983 vey (UGS) Guidelines for Evaluating Landslide HazardsRudd Canyon debris flow in Farmington deposited in Utah (Hylland, 1996) and Guidelines for Evaluatingapproximately 90,000 cubic yards of sediment on the Surface-Fault-Rupture Hazards in Utah (Christenson andalluvial fan, damaged 35 houses, and caused an estimated others, 2003). The geologist has the responsibility to (1)$3 million in property damage (Deng and others, 1992). conduct a study that is thorough and cost effective, (2) be Variations in sediment-water concentrations produce familiar with and apply appropriate investigation meth-a continuum of sediment-water flow types that build allu- ods, (3) record accurate observations and measurements,vial fans. Beverage and Culbertson (1964), Pierson and (4) use proper judgment, and (5) present valid conclu-Costa (1987), and Costa (1988) describe the following sions and recommendations supported by adequate dataflow types based on generalized sediment-water concen- and sound interpretations. The geologist must also under-trations and resulting flow behavior: stream flow (less stand and clearly state the uncertainties and limitations ofthan 20% sediment by volume), hyperconcentrated flow the investigative methods used and the uncertainties asso-(20 to 60% sediment by volume), and debris flow (greater ciated with design-parameter estimates.than 60% sediment by volume). These categories areapproximate because the exact sediment-water concentra- Limitationstion and flow type depend on the grain-size distributionand physical-chemical composition of the flows. Also, These guidelines identify important issues and gener-field observations and video recordings of poorly sorted al methods for evaluating debris-flow hazards; they dowater-saturated sediment provide evidence that no unique not discuss all methods and are not a step-by-step primerflow type adequately describes the range of mechanical for hazard evaluation. The level of detail appropriate forbehaviors exhibited by these sediment flows (Iverson, a particular evaluation depends on several factors, includ-2003). All three flow types can occur during a single ing the type, nature, and location of proposed develop-event. The National Research Council (1996) report on ment; the geology and physical characteristics of theAlluvial-Fan Flooding considers stream, hyperconcen- drainage basin, feeder channel, and alluvial fan; thetrated, and debris-flow types of alluvial-fan flooding. record of previous debris flows; and the level of riskThe term debris flood has been used in Utah to describe acceptable to property owners and land-use regulators. Ahyperconcentrated flows (Wieczorek and others, 1983). uniform level of acceptable risk for debris flows based on These guidelines address only hazards associated recurrence or frequency/volume relationships, such as thewith hyperconcentrated- and debris-flow sediment-water 100-year flood or the 2% in 50-year exceedance proba-concentrations and not stream-flow flooding on alluvial bility for earthquake ground shaking, has not been estab-fans. The term debris flow is used here in a general way lished in Utah.to include all flows within the hyperconcentrated- and Historical records of sedimentation events in Utahdebris-flow sediment-water concentration range. These indicate that debris flows are highly variable in terms ofare the most destructive flows, and it can be difficult to size, material properties, and travel and depositionaldistinguish between hyperconcentrated and debris flows behavior; therefore, a high level of precision for debris-based on their deposits. Stream flow involves sediment flow design parameters cannot yet be attained. Conse-transport by entrained bed load and suspended sediment quently, prudent design parameters and engineeringload associated with water transport. Sheetfloods are designs must be used where risk reduction is necessary.unconfined stream flows that spread over the alluvial fan Appropriate disclosure of the debris-flow-hazard evalua-Guidelines for the geologic evaluation of debris-flow hazards on alluvial fans in Utah 3

tion to future property owners is also advisable. Information Sources

The state-of-the-art of debris-flow-hazard evalua-tion continues to evolve as our knowledge of sediment- Sources of information for debris-flow-hazard evalu-flow processes advances. As new techniques become ations include U.S. Geological Survey and UGS mapsavailable and generally accepted they should be used in that show debris-flow source areas at a nationwide scalefuture hazard evaluations. Ranges for debris-flow bulk- (1:2,500,000; Brabb and others, 2000), statewide scaleing rates, flow volumes, runout distances, deposit areas, (1:500,000; Brabb and others, 1989; Harty, 1991), and 30and deposit thicknesses have not been established and x 60-minute quadrangle scale (1:100,000; UGS Open-further research is necessary to quantify the physical File Reports) for the entire state. The 30 x 60-minutecharacteristics of debris flows in Utah. The methods out- quadrangle maps show both the source and depositionallined in these guidelines are considered to be practical areas of some historical debris flows. Alluvial-fan de-and reasonable methods for obtaining planning, design, posits are commonly shown on modern geologic maps,and risk-reduction information, but these methods may and the UGS and others map surficial (Quaternary) geol-not apply in all cases. The user is responsible for under- ogy on 712 - minute scale quadrangle maps (1:24,000).standing the appropriateness of the various methods and Wasatch Front counties have maps available in countywhere they apply. planning offices showing special-study areas where debris-flow-hazard evaluations are required. Surficial geologic maps generally show alluvial-fan deposits of DEBRIS-FLOW-HAZARD EVALUATION different ages and differentiate stream alluvium from alluvial-fan deposits. A debris-flow-hazard evaluation is necessary when Numerous investigators have studied debris-flowdeveloping on active alluvial fans where relatively recent processes and performed debris-flow-hazard evaluationsdeposition has occurred. The evaluation requires appli- in Utah. Many studies address the 1983 and 1984 debriscation of quantitative and objective procedures to esti- flows that initiated during a widespread rapid-snowmelt period. Christenson (1986) discusses mapping, hazardmate the location and recurrence of flows, assess their evaluation, and mitigation measures following the debrisimpacts, and provide recommendations for risk-reduction flows of 1983. Wieczorek and others (1983, 1989)measures if necessary. The hazard evaluation must state described the potential for debris flows and debris floodsthe intended land use because site usage has direct bear- and mitigation measures along the Wasatch Fronting on the degree of risk to people and structures. The between Salt Lake City and Willard. Lips (1985, 1993)UGS recommends critical facilities and structures for mapped 1983 and 1984 landslides and debris flows inhuman occupancy not be placed in active debris-flow central Utah. Cannon (1989) evaluated the travel-dis-travel and deposition areas unless methods are used to tance potential of debris flows that occurred in 1983 andeither eliminate or reduce the risk to an acceptable level. 1984. Paul and Baker (1923), Woolley (1946), CroftIn some cases, risk-reduction measures may be needed to (1967), Butler and Marsell (1972), Marsell (1972), andprotect existing development. Keate (1991) provide documentation and photographs of To evaluate the hazard on active alluvial fans, the fre- historical debris flows and flooding in Utah prior to thequency, volume (deposit area and thickness), and runout 1983 events.distance of past debris flows must be determined. The Several researchers investigated different aspects ofgeologic methods presented here rely on using the geo- the 1983 and 1984 Davis County debris flows. Packlogic characteristics of existing alluvial-fan deposits as (1985), for the purpose of landslide susceptibility map-well as drainage-basin and feeder-channel sediment-sup- ping, used a multivariate analysis to evaluate factorsply conditions to estimate the characteristics of past related to initiation of debris slides in 1983 that thendebris flows. Historical records can provide direct evi- transformed into debris flows. Pierson (1985) describeddence of debris-flow volume, frequency, and deposition- flow composition and dynamics of the 1983 Ruddal area. The observation period in Utah is short, and Canyon debris flow in Farmington. Santi (1988) studieddebris flows either have not occurred or have not been the kinematics of debris-flow transport and the bulking ofdocumented. Therefore, geologic methods provide the colluvium and channel sediment during a 1984 debrisprincipal means of determining the history of debris-flow flow in Layton. Mathewson and others (1990) studiedactivity on alluvial fans. Multiple geologic methods bedrock aquifers and the location of springs and seepsshould be used whenever possible to compare results of that initiated colluvial slope failures in 1983 and 1984different methods to understand the appropriateness, that then transformed into debris flows. Keaton (1988)validity, and limitations of each method and increase con- and Keaton and others (1991) developed a probabilisticfidence in the hazard evaluation. model to assess debris-flow hazards on alluvial fans. Where stream flow dominates on an alluvial fan a Williams and Lowe (1990) estimated channel sedimentstream-flow-flooding evaluation is necessary, but a bulking rates by comparing cross-channel profiles ofdebris-flow-hazard evaluation is not required. The channels that discharged historical debris flows withNational Research Council (1996) report on Alluvial-Fan channels that had not discharged flows in historical time.Flooding and the Federal Emergency Management Deng and others (1992) studied debris-flow impactAgency (1999) Guidelines for Determining Flood Haz- forces, types of house damage, and economic losses fromards on Alluvial Fans provide guidance for evaluating the the 1983 Rudd Canyon debris flow. Coleman (1995)stream-flow component of alluvial-fan flooding. studied the possible role of watershed terraces in con-4 Utah Geological Survey

tributing material to the 1983-84 debris flows. Ala (1995) oldest and most recent photos available is useful to eval-studied the interaction of bedrock structure, lithology, and uate drainage-basin and alluvial-fan changes throughground water and their combined influence on colluvial time. Obtaining aerial photographs taken after historicalslope failures that generated debris flows. Skelton (1995) debris flows allows direct mapping of sediment sourcesstudied the geologic control of seeps and springs in the and deposits.Farmington Canyon Complex and their role in generatingcolluvial slope failures. Eblen (1995) modeled colluvialslope stability to understand the initiation of the 1983 Alluvial-Fan Evaluationslope failures that mobilized into debris flows. Alluvial fans are landforms composed of a complex Outside of Utah others have outlined approaches for assemblage of debris-, hyperconcentrated-, and stream-evaluating debris-flow hazards and methods for estimat- flow deposits. Alluvial-fan geomorphology, sedimentol-ing design parameters for debris-flow-risk reduction. ogy, and stratigraphy provide a long-term depositionalHungr and others (1984) described approaches to esti- history of the frequency, volume, and depositional behav-mate debris-flow frequency, volume, peak discharge, ior of past flows, and provide a geologic basis for esti-velocity, and runout distance in western Canada. Van- mating debris-flow hazards.Dine (1985) described conditions conducive to debrisflows, triggering events, effects, and mitigation in thesouthern Canadian Cordillera. Hungr and others (1987) Defining the Active-Fan Areadescribed debris-flow-engineering concepts and risk The first step in an alluvial-fan evaluation is deter-reduction in source, transport, and deposition zones in mining the active-fan area using mapping and alluvial-fanBritish Columbia. Jackson (1987) outlined methods for dating techniques. The active-fan area is where relative-evaluating debris-flow hazards on alluvial fans in the ly recent deposition, erosion, and alluvial-fan floodingCanadian Rocky Mountains based on the presence of have occurred (figure 2). In general, sites of sedimentdebris-flow deposits, alluvial-fan geomorphic features, deposition during Holocene time (past 10,000 years;deposit ages, debris-flow frequency, and basin condi- post-Lake Bonneville in northwest Utah) are consideredtions. Jackson (1987) also provided a flow chart summa- active unless proven otherwise. Aerial photographs, de-rizing debris-flow-hazard evaluation. Jackson and others tailed topographic maps, and field verification of the(1987) used geomorphic and sedimentologic criteria to extent, type, character, and age of alluvial-fan depositsdistinguish alluvial fans prone to debris flows and those are used to map active-fan areas. The youngest debris-dominated by stream-flow processes. Ellen and others flow deposits are generally indicative of debris flows pro-(1993) used digital simulations to map debris-flow haz- duced during the modern climate regime and are impor-ards in the Honolulu District of Oahu, Hawaii. VanDine tant for estimating the likely volume and runout for future(1996) summarized the use of debris-flow control struc- flows. The active fan is often used as a zoning tool totures for forest engineering applications in British identify special-study areas where detailed debris-flow-Columbia. Boyer (2002) discussed acceptable debris- hazard evaluations are required prior to development.flow-risk levels for subdivisions in British Columbiaand provided a suggested outline for debris-flowstudies on alluvial fans. U.S. Natural Resources Conservation Service(NRCS, formerly Soil Conservation Service) soilsurveys show soils on alluvial fans and in drainagebasins. These soil surveys provide information onsoil type, depth, permeability, erodibility, slopesteepness, vegetation, and parent material. Some soilsurveys document historical debris-flow activity. Newspaper articles and event reports often pro-vide descriptions of historical debris flows and pho-tographs showing impacts on developed areas. Writ-ten observations and photographs of historical debrisflows provide useful information on flow volume,flow velocity, flow depth, deposit thickness, depositareas, and building damage. Comparison of histori-cal debris-flow deposits with prehistoric depositsallows the geologist to check if the historical debrisflow is a typical event relative to other flows pre-served in the sedimentary record. Stereoscopic aerial photographs are a fundamen-tal tool for evaluating drainage basins and alluvialfans. Interpretation of aerial photographs can pro-vide information on surficial geology, soils, bedrockexposures, channel characteristics, landslides, previ- Figure 2. Active and inactive fans, feeder channel, and intersection point.ous debris flows, relative deposit ages, erosional Modified from Bull (1977). Reproduced with permission by Edwardareas, land use, and vegetation types. Reviewing the Arnold (Publishers) Ltd., London.Guidelines for the geologic evaluation of debris-flow hazards on alluvial fans in Utah 5

The National Research Council (1996) report on Alluvial- tal fan areas, debris flows generally have lower velocitiesFan Flooding provides criteria for differentiating active and shallower flow depths and deposits, and therefore areand inactive alluvial fans. less destructive. Often, distal fan areas are dominated by stream-flow processes only. However, some debris flowsMapping Alluvial-Fan and Debris-Flow Deposits may create their own channels by producing levees on the fan and conveying sediment farther downfan, or blocking Geologic mapping is critical for identifying and the active channel and avulse (make an abrupt change indescribing the active areas of alluvial fans. Mapping of course) to create new channels. Unpredictable flow be-debris-flow and other deposits generally focuses on land- havior is typical of debris flows and must be consideredforms; the extent, type, character, and age of geologic when addressing debris-flow depositional areas, runoutdeposits, specifically individual debris flows; and strati- distances, and depositional behavior on alluvial fans.graphic relations between deposits. Peterson (1981), The proximal part of an alluvial fan is generallyChristenson and Purcell (1985), Wells and Harvey made up of vertically stacked debris-flow lobes and lev-(1987), Bull (1991), Whipple and Dunne (1992),Doelling and Willis (1995), Hereford and others (1996), ees that result in thick and coarse deposits that exhibit theand Webb and others (1999) provide examples and sug- roughest surface on the fan (figure 3). Hyperconcentrat-gestions for mapping alluvial-fan deposits. ed flows may be interbedded with debris flows in the The geomorphic, sedimentologic, and stratigraphic proximal fan area, but are generally thinner and haverelations recognized during mapping of alluvial-fan smoother surfaces due to their higher initial water con-deposits provide insight into debris-flow recurrence, vol- tent. Proximal fan deposits generally transition to thinnerumes, depositional behavior, and therefore debris-flow and finer grained deposits downfan, resulting in smootherhazard in the proximal, medial, and distal fan areas (fig- fan surfaces in medial and distal fan areas (figure 3).ure 3). The intersection point or apex of the active fan is Coarser grained sedimentary facies grade downfan intowhere the feeder channel ends and sediment flows lose finer grained facies deposited by more dilute sedimentconfinement and can spread laterally, thin, and deposit flows. The downfan decrease in grain size generally cor-sediment (figure 2; Blair and McPherson, 1994). Most responds with a decrease in fan-slope angle. Coarserfeeder channels lose confinement on the upper fan, but grained debris-flow deposits generally create steeperothers may incise the inactive upper fan and convey sed- proximal-fan slopes (6-8) while finer grained stream-iment and flood flows farther downfan via a fanhead flow deposits form gentle distal-fan slopes (2-3) (Na-trench or channel (figure 2). tional Research Council, 1996). In proximal fan areas, debris flows generally have the Differences in bedding, sediment sorting, grain size,highest velocity and greatest flow depth and deposit and texture are useful to distinguish debris-, hypercon-thickness, and are therefore the most destructive. In dis- centrated-, and stream-flow deposits. Costa and Jarrett

(1981, p. 312-317), Wells and Harvey (1987, p. 188), ular location. Numerical dating techniques includeCosta (1988, p. 118-119), Harvey (1989, p. 144), the sequential photographs, historical records, dating the ageNational Research Council (1996, p. 74), and Meyer and of vegetation, and isotopic dating, principally radiocar-Wells (1997, p. 778) provide morphologic and sedimen- bon. Radiocarbon ages of paleosols buried by debristologic criteria (surface morphology, internal structures, flows can provide closely limiting maximum ages of thetexture, grain size, and sorting) for differentiating the overlying flow (Forman and Miller, 1989). Radiocarbonthree flow types. In general, debris-flow deposits are ages of detrital charcoal within a debris-flow deposit pro-matrix supported and poorly sorted, hyperconcentrated- vide a general limiting maximum age. The applicabilityflow deposits are clast supported and poorly to moderate- and effectiveness of radiocarbon dating of debris-flowly sorted, and stream-flow deposits are clast supported events is governed by the presence and type of datableand moderately to well sorted. Table 1 is modified from material and available financial resources (Lettis and Kel-Costa (1988) and shows geomorphic and sedimentologic son, 2000).characteristics of debris-, hyperconcentrated-, andstream-flow deposits. Grain-size analysis is useful in Subsurface Explorationclassifying deposits into the different flow types (Pierson,1985). Subsurface exploration using test pits, trenches, and More than one flow type may occur during a sedi- natural exposures is useful in obtaining sedimentologicmentation event. Keaton (1988) described an ideal verti- and stratigraphic information regarding previous debriscal alluvial-fan stratigraphic sequence based on deposits flows. Test-pit and trench excavations can provide infor-in Davis County and published eyewitness accounts. The mation on flow type, thickness, the across- and downfanideal sequence resulting from a single debris flow con- extent of individual flows, and volume based on thicknesssists of a basal plastic debris-flow deposit, sequentially and area. The type, number, and spacing of excavationsoverlain by a viscous debris-flow, hyperconcentrated- depend on the purpose and scale of the hazard investiga-flow, and finally a stream-flow deposit owing to time- tion, geologic complexity, rate of downfan and across-fanvarying availability of sediment and water. Janda and transitions in flow type and thickness, and anticipatedothers (1981) identified a similar vertical sequence in risk-reduction measures. T-shaped test pits or trenchesdebris-flow deposits at Mount St. Helens, Washington allow determination of three-dimensional deposit rela-and attributed the vertical sequence to rapid transitions tionships. Excavations in the proximal fan areas general-between flow types. ly need to be deeper due to thicker deposits. To evaluate the entire fan, tens of excavations may be required.Determining the Age of Debris-Flow Deposits Mulvey (1993) used subsurface stratigraphic data from seven test pits to estimate flow types, deposit thick- Both relative and numerical techniques (Noller and nesses, the across- and downfan extent of deposits,others, 2000) are useful for dating debris-flow deposits deposit volumes, and age of deposits to interpret theand determining the frequency of past debris flows on a depositional history of a 2-acre post-Bonneville fan infan. Relative dating methods include boulder weathering, Centerville. Blair and McPherson (1994) used across-rock varnish, soil-profile development (including pedo- and downfan stratigraphic cross sections to display, ana-genic carbonate accumulation), lichen growth, and vege- lyze, and interpret the surface and subsurface interrela-tation age and pattern. The amount of soil development tionships of fan slope, deposit levees and lobes, depositon a buried debris-flow surface is an indicator of the rel- and sediment facies, and grain size. However strati-ative amount of time between debris flows at that partic- graphic interpretation can be problematic. Debris-flow

Table 1. Geomorphic and sedimentologic criteria for differentiating water and sediment flows (modified from Costa, 1988).

deposits in a sedimentary sequence that have similar During the drought years of 1999-2004 in northerngrain sizes and lack an intervening paleosol or other dis- Utah, 26 debris flows occurred in 7 wildfire areas, includ-tinct layer can be difficult to distinguish. The lack of dis- ing repeated flows from single drainages in differenttinction between individual debris-flow deposits can lead storms and multiple flows from different drainages dur-to underestimating debris-flow recurrence and overesti- ing the same storm. Debris-flow-hazard evaluations fol-mating debris-flow magnitude (Major, 1997). lowing a wildfire address burn severity and hillslope and channel conditions. Cannon and Reneau (2000) provide methods for evaluating debris-flow susceptibility follow- Drainage-Basin and Channel Evaluation ing wildfires. Evanstad and Rasely (1995) and the U.S. Drainage-basin and channel evaluations determine Natural Resources Conservation Service (2000) estimat-the conditions and processes that govern sediment supply ed fire-related hillslope sediment yield for Wasatch Frontand transport to the fan surface, and provide an inde- drainages in Davis and Weber Counties and the lowerpendent check of alluvial-fan evaluations. Drainage- Provo River drainage basin in Utah County. However,basin and channel evaluation involves estimating the ero- their sediment volume estimates are for annual post-burnsion potential of the basin and feeder channel and the vol- hillslope sediment yields only and do not include channelume, grain size, and gradation of sediment that could be sediment bulking that must be considered when estimat-incorporated into a debris flow. The evaluation also con- ing total debris-flow volumes. Wells (1987), Florsheimsiders different debris-flow initiation mechanisms. The and others (1991), Cannon and others (1995), Meyer andresults of the drainage-basin and channel evaluation are others (1995), Cannon and Reneau (2000), Kirkham andused to estimate the probability of occurrence and design others (2000), Robichaud and others (2000), and Cannonvolumes of future debris flows. In some cases, evaluation (2001) discuss post-burn conditions and debris-flow sus-of the drainage basin and channel may be performed ceptibility following wildfires.independently of the alluvial-fan evaluation. For exam-ple, a wildfire in a drainage basin may initiate a post-burn Debris-Flow Susceptibility of the Basinanalysis of the drainage basin and channels to estimate orrevise the erodible sediment volume and the probability Debris-flow susceptibility is related to the erosionof post-fire debris flows. and landslide potential of drainage-basin slopes and the volume of erodible sediment stored in drainage-basin channels. Characterizing drainage-basin morphologicDebris-Flow Initiation parameters, mapping bedrock and surficial geology, and Debris flows initiate in the drainage basin and require estimating the volume of erodible channel sediment pro-a hydrologic trigger such as intense or prolonged rainfall, vides information on the likelihood and volume of futurerapid snowmelt, and/or ground-water discharge. Intense debris flows.thunderstorm rainfall, often referred to as cloudburst Important basin parameters include area, relief, andstorms by early debris-flow investigators in Utah (Wool- length and gradient of channels. A description of theley, 1946; Butler and Marsell, 1972), has generated types and density of vegetation and land use providesnumerous debris flows. Conditions in the drainage basin information on the possible effects of wildfire and landimportant in initiating debris flows are the basin relief, use on surface-water runoff and erosion. Small, steepchannel gradient, bedrock and surficial geology, vegeta- drainage basins are well suited for generating debristion and wildfire, and land use. Exposed bedrock on hill- flows because of their efficiency in concentrating andsides promotes rapid surface-water runoff, which helps accelerating overland surface-water flow.generate debris flows. Wildfires can destroy vegetation Both surficial and bedrock geology play a role in theand may also create water-repellent soils that result in susceptibility of drainage basins to produce flows. Somerapid runoff. All of these conditions work in combination bedrock weathers rapidly and provides an abundant sedi-to promote debris flows. ment supply, whereas resistant bedrock supplies sediment In Utah, above-normal precipitation from 1980 at a slower rate. Exposed cliff-forming bedrock greatlythrough 1986 produced numerous snowmelt-generated increases runoff.landslides (mostly debris slides) that transformed into Some bedrock, such as shale, weathers and providesdebris flows and then traveled down channels (Brabb, fine-grained clay-rich sediment, whereas other bedrock1989; Harty, 1991). Many of these debris flows occurred types provide mostly coarse sediment. The clay contentduring periods of rapid snowmelt and high stream flows, of debris flows directly influences flow properties. Costawhen Santi (1988) indicates that saturated channel sedi- (1984) states that small changes (1 to 2%) in clay contentment is more easily entrained into debris flows. in a debris flow can greatly increase mobility due to In contrast to wet climate conditions, dry conditions reduced permeability and increased pore pressure. Theoften lead to wildfires that partially or completely burn presence of silt and clay in a slurry aids in maintainingdrainage-basin vegetation, creating conditions for high pore pressure to enhance the potential flow mobilityincreased runoff and erosion. Intense thunderstorm rain- and runout (Iverson, 2003).fall on steep burned slopes may produces debris flows. Surficial geologic deposits that influence the sedi-Relatively small amounts of intense thunderstorm rainfall ment supply include (1) colluvium on steep slopes sus-(a few tenths of an inch per hour) are capable of trigger- ceptible to forming debris slides, (2) partially detacheding fire-related debris flows (McDonald and Giraud, shallow landslides, (3) foot-slope colluvium filling the2002; Cannon and others, 2003). drainage basin channel that may contribute sediment by8 Utah Geological Survey

bank erosion and sloughing, and (4) stream-channel allu- from the channel (Croft, 1967; Santi, 1988; Keaton andvium. Lowe, 1998). Most estimates of potential sediment bulk- Mapping debris slides in a drainage basin and deter- ing are based on a unit-volume analysis of erodible sedi-mining their potential to transform into debris flows is ment stored in the channel, generally expressed in cubicimportant in evaluating debris-flow susceptibility. Most yards per linear foot of channel (Hungr and others, 1984;of the 1983-84 debris flows along the Wasatch Front ini- VanDine, 1985; Williams and Lowe, 1990). The sedi-tiated as shallow debris slides in steep colluvial slopes ment volume stored in individual relatively homogeneousbelow the retreating snowline (Anderson and others, channel reaches is estimated, and then the channel-reach1984; Pack, 1985). Aerial-photo analysis can show col- volumes are summed to obtain a total volume. The totalluvium on steep slopes and previous debris slides or par- channel volume is an upper bound volume and needs totially detached debris slides. A literature search of histor- be compared to historical (VanDine, 1996) and mappedical debris slides in the area and in areas of similar geol- alluvial-fan flow volumes to derive a design volume. Ifogy will help to identify debris-slide susceptibility. For easily eroded soils and slopes prone to landsliding areexample, documented relations exist between debris present, then appropriate volumes for landslide and hills-slides and debris flows in drainage basins in the Precam- lope contributions determined from other drainage-basinbrian Farmington Canyon Complex of Davis County landslide volumes should be added to the channel vol-(Pack, 1985) and in the Tertiary-Cretaceous rocks of the ume.Wasatch Plateau (Lips, 1985). Estimating a potential sediment-bulking rate requires Drainage basins that experience rapid snowmelt field inspection of the drainage basin and channels. Mea-events have an increased debris-flow hazard. Pack suring cross-channel profiles and estimating the erodible(1985), Mathewson and others (1990), and Eblen (1995) depth of channel sediment is necessary to estimate thedetermined that in the 1983-84 Davis County debris sediment volume available for bulking (figure 4). Evenflows, water infiltration into fractured bedrock aquifers though a great deal of geologic judgment may be requiredfrom rapid snowmelt contributed to increased pore-water to make the volume estimate, this is probably the mostpressure in steep colluvial slopes that triggered localized reliable and practical method for bedrock-floored chan-colluvial landslides (debris slides) that transformed into nels. The design volume should not be based solely ondebris flows. Santi (1988) suggested that sediment bulk- empirical bulking of specific flood flows (for example,ing is more likely when passage of a debris flow occurs bulking a 100-year flood with sediment) because empiri-during periods of stream flow and associated saturated cal bulking does not consider shallow landslide-generat-channel sediment, and will result in larger debris-flow ed debris flows (National Research Council, 1996), chan-volumes. nel bedrock reaches with no stored sediment, and the typ- Wieczorek and others (1983, 1989) used ground- ically longer recurrence period of debris flows. Thewater levels, the presence of partially detached landslide channel inspection should also provide a description ofmasses, and estimates of channel sediment bulking to the character and gradation of sediment and wood debrisevaluate debris-flow potential along the Wasatch Front that could be incorporated into future debris flows.between Salt Lake City and Willard. Superelevated lev- Hungr and others (1984), VanDine (1985), andees, mud lines, and trim lines along channels are evidence Williams and Lowe (1990) use historical flow volumesof peak discharge. Measurements from these features are and channel sediment bulking rates to estimate potentialuseful in estimating velocity and peak flow (Johnson and debris-flow volumes. Williams and Lowe (1990), fol-Rodine, 1984). Determining the age of vegetation grow- lowing the 1983 debris flows in Davis County, compareding on the levees provides a minimum age of past debris- cross-channel profiles of drainages that had dischargedflow activity. historical debris flows with those that had not to estimate Land use and land-use changes within a drainage the amount of channel sediment bulked by historicalbasin may also influence debris-flow susceptibility. Land flows. They estimated an average bulking rate of 12development often creates impervious surfaces that cubic yards per linear foot (yd3/ft) of channel for histori-increase the rate and volume of runoff. Development cal debris flows and used it to estimate flow volumes formay also remove vegetation and expose soils, promoting drainage basins without historical debris flows, but rec-erosion, increasing sediment yield, and decreasing natu- ommended using this estimate only for perennial streamsral slope stability within the drainage basin. Debris-flow- in Davis County. Bulking rates for intermittent andhazard evaluation must address development-induced ephemeral streams are generally lower. For example,conditions where applicable. Mulvey and Lowe (1992) estimated a bulking rate of 5 yd3/ft for the 1991 Cameron Cove debris flow in DavisChannel Sediment Bulking and Flow-Volume Esti- County. Some of the fire-related debris flows at the 2002mation Dry Mountain/Santaquin event (McDonald and Giraud, 2002) have estimated bulking rates of 1.5 yd3/ft of Sediment supply, erosion conditions, and hydrologic ephemeral channel. Hungr and others (1984), VanDineconditions of the drainage basin and channel determine (1985, 1996), and Williams and Lowe (1990) all con-the sediment and water concentration (flow type) and cluded that channel length and channel sediment storageflow volume that reaches an alluvial fan. Estimating are the most important factors in estimating future debris-channel sediment volume available for entrainment or flow volumes.bulking is critical because study of historical debris flows Some drainage basins may have recently dischargedindicates 80 to 90% of the debris-flow volume comes a debris flow leaving little sediment available in the feed-Guidelines for the geologic evaluation of debris-flow hazards on alluvial fans in Utah 9

Figure 4. Channel sediment and cross section used to estimate sediment volume available for bulking. (a) Channel erosion from the Sep-tember 10, 2002, fire-related debris flow on Dry Mountain east of Santaquin, Utah. The solid line shows the eroded channel after the debrisflow, the dashed line shows the estimated channel prior to debris-flow passage. (b) Sketch of channel cross-section showing stored channelsediment above bedrock. The dashed line shows the estimated upper bound width and depth of channel sediment available for sediment bulk-ing.

er channel for sediment bulking for future debris flows. DEBRIS-FLOW-RISK REDUCTIONKeaton and others (1991) state that channels with recentdebris flows will discharge future flows of less volume Eisbacher and Clague (1984), Hungr and othersuntil the feeder channel has recharged with sediment. In (1987), and VanDine (1996) group debris-flow-riskthese situations an evaluation must consider remaining reduction into two categories: passive and active. Passivechannel sediment as well as the rate of sediment recharge methods involve avoiding debris-flow-hazard areas eitherto the channel (National Research Council, 1996). The permanently or at times of imminent danger. Passivepercent of channel length lined by bedrock is a distinct methods do not prevent, control, or modify debris flows.indication of the volume of sediment remaining because Active methods modify the hazard using debris-flow-sediment cannot be scoured from bedrock reaches. control structures to prevent or reduce the risk. TheseWilliams and Lowe (1990) suggest that in Davis County debris-flow-control structures require engineering designthe drainage basins capable of producing future large de- using appropriate geologic inputs. In terms of develop-bris flows are basins that have not discharged historical ment on alluvial fans, active risk-reduction measures withdebris flows. However, drainage basins having a limited control structures generally attempt to maximize thedebris-flow volume potential due to lack of channel sedi- buildable space and provide a reasonable level of protec-ment may still have a high stream-flow-flooding potential. tion.10 Utah Geological Survey

Hungr and others (1987) and VanDine (1996) divide vary widely among alluvial fans and few data exist todebris-flow-control structures along lower channel reach- quantify debris-flow frequency-volume relations. Otheres and on alluvial fans into two basic types: open struc- difficulties in developing and using probabilistic modelstures (which constrain flow) and closed structures (which in debris-flow-hazard assessment include:contain debris). Examples of open debris-flow-controlstructures include unconfined deposition areas, impedi- Frequencies are time-dependent. Many drain-ments to flow (baffles), check dams, lined channels, lat- ages must recharge channel sediment followingeral walls or berms, deflection walls or berms, and termi- a large-volume debris flow; the size and prob-nal walls, berms, or barriers. Examples of closed debris- ability of future debris flows depend on the sizeflow-control structures include debris racks, or other of and time since the last event.forms of debris-straining structures located in the chan-nel, and debris barriers and associated storage basins with Statistically based cloudburst storms typicallya debris-straining structure (outlet) incorporated into the used for stream-flooding evaluations (for ex-design. ample, the 100-year storm) are not applicable In Utah, engineered sediment storage basins are the to debris-flow models because debris-flow dis-most common type of control structure used to reduce charges do not relate directly to flood dis-debris-flow risks. These structures generally benefit the charges, and in Utah many debris flows arecommunity as well as the individual subdivider or caused by rapid snowmelt rather than cloud-landowner, but they are typically expensive, require peri- burst storms.odic maintenance and sediment removal, and must oftenbe located in areas not owned or controlled by an indi- Wildfires and land-use changes in the drainagevidual subdivider. For these reasons, debris-flow- and basin introduce significant uncertainty be-flood-risk-reduction structures are commonly govern- cause they can temporarily greatly increase de-ment public-works or shared public-private responsibili- bris-flow probabilities.ties, rather than solely a subdivider or landowner respon-sibility. This is particularly true in urban settings where Because of these complexities, generally acceptedthe delineated hazard area may include more than one return periods for design of debris-flow risk-reductionsubdivision and other pre-existing development. In some measures based on probabilistic models do not exist,cases, local flood-control agencies such as Davis County unlike for earthquake ground shaking and flooding,Flood Control manage both debris-flow and stream- which have established design return periods of 2,500flooding hazards. years (International Building Code) and 100 years (FEMAs National Flood Insurance Program), respective- ly. Although Keaton (1988) and Keaton and others DESIGN CONSIDERATIONS FOR (1991) developed a probabilistic model for debris flows RISK REDUCTION in Davis County where a relatively complete record of historical debris flows exists, the high degree of irregu- The debris-flow hazard at a particular site depends on larity and uncertainty in return periods limited theirthe sites location on the alluvial fan. Both debris-flow results and the practical application of their model. Inimpact and sediment burial are more likely and of greater some cases rather than assigning an absolute probabilitymagnitude in proximal fan areas than in medial and distal of debris-flow occurrence, many debris-flow practition-fan areas (figure 3). Decisions regarding acceptable risk ers assign a relative probability of occurrence (VanDine,and appropriate control-structure design involve weigh- 1996) based on frequencies in similar basins and fans ining the probability of occurrence in relation to the conse- the geographic areas that have experienced historicalquences of a debris flow and the residual risk level after debris flows.implementing risk-reduction measures. Therefore, haz- The UGS believes Holocene-age (past 10,000 years)ard evaluations estimate the likely size, frequency, and debris-flow deposits on an alluvial fan are sufficient evi-depositional area of debris flows on an alluvial fan as dence to recommend site-specific, debris-flow-hazardaccurately as possible. studies and appropriate implementation of risk-reduction measures. Holocene deposits were deposited under cli- matic conditions similar to the present and therefore indi- Considering Frequency in Design cate a current hazard unless geologic and topographic The frequency of past debris flows on an alluvial fan conditions on the alluvial fan have changed. If site-spe-is a fundamental indicator of future debris-flow activity. cific data on debris-flow recurrence are sufficient toTo address the past frequency of debris flows, detailed develop a probabilistic model, then the model may begeologic studies involving geochronology are generally used in consultation with local government regulators torequired. Little or nothing is known about the past fre- help determine an appropriate level of risk reduction.quency of debris flows on most alluvial fans in Utah.Studies by Keaton (1988), Lips (1993), and Mulvey Debris-Flow-Hazard Zones(1993) indicate that large, destructive debris flows on thealluvial fans they studied have return periods of a few Debris-flow-hazard zones identify potential impactshundred to thousands of years. However, return periods and associated risks, help determine appropriate risk-Guidelines for the geologic evaluation of debris-flow hazards on alluvial fans in Utah 11

reduction measures, and aid in land-use planning deci- laboratory methods to predict slurry characteristics basedsions. Hungr and others (1987) outline three debris-flow- on sedimentology and stratigraphy of alluvial-fanhazard zones: (1) a direct impact zone where high-energy deposits. Flow characteristics are also important to helpflows increase the risk of impact damage due to flow estimate associated water volume.velocity, flow thickness, and the maximum clast size; (2) Estimating debris-flow volume is necessary wherean indirect impact zone where impact risk is lower, but debris storage basins are planned. Because debris-flowwhere damage from sediment burial and debris-flow and behavior is difficult to predict and flows difficult to route,water transport is high; and (3) a flood zone potentially debris storage basins and deflection walls or berms areexposed to flooding due to channel blockage and water common methods of debris-flow risk reduction. Thedraining from debris deposits. These zones roughly routing of debris flows off an alluvial fan is a difficult andequate to proximal, medial, and distal fan areas, respec- complex task. OBrien and Julien (1997) state that chan-tively (figure 3). Historical debris-flow records, deposit nel conveyance of debris flows off an alluvial fan is notcharacteristics, and detailed topography are required to recommended unless the situation is appropriate becauseoutline these hazard zones. Site-specific studies are there are numerous factors that can cause the flow to plugrequired to define which zone applies to a particular site the conveyance channel. Debris basins typically captureand to determine the most appropriate land use and risk- sediment at the drainage mouth before the debris flowreduction techniques to employ. travels unpredictably across the alluvial fan. For debris basin capacity, the thickness and area of individual flows on the alluvial fan and erodible channel sediment vol- Estimating Geologic Parameters for umes are needed to estimate design debris volumes. Esti- Engineering Design mates of sediment stored in channels are usually maxi- mum or worst-case volumes that represent an upper Geologic estimates of debris-flow design parameters volume limit. Channel estimates may exceed the alluvial-are necessary for engineering design of risk-reduction fan estimates because typically not all channel sedimentstructures. The most appropriate data often come from is eroded and deposited on the fan, and the channel esti-historical or late Holocene debris flows that can be mate includes suspended sediment transported off the fanmapped on the fan surface. Flow and deposit character- by stream flows. Conversely, the alluvial-fan estimateistics are also necessary to estimate peak discharge and may exceed the channel estimate if a recent large flow hascalibrate computer-based hydraulic flow routing models removed most channel sediment. VanDine (1996) con-(OBrien and Julien, 1997). siders the design volume to be the reasonable upper limit Geologic parameters required for engineering design of material that will ultimately reach the fan.vary depending on the risk-reduction structure proposed. Flow volume is also important in modeling runoutEngineering designs for debris-flow risk-reduction struc- and deposition. OBrien and Julien (1997), in theirtures are site specific (VanDine and others, 1997), and hydraulic modeling of debris-flow runout, emphasize thegenerally involve quantifying specific fan, feeder chan- importance of making conservative estimates of the avail-nel, deposit, and flow parameters. Geomorphic fan able volume of sediment in the drainage basin, and com-parameters include areas of active deposition, surface paring that volume to alluvial-fan deposit volumes togradients, surface roughness (channels, levees, lobes), determine an appropriate modeling volume.and topography. Feeder channel parameters include Geologic design parameters are also needed for thechannel gradient, channel capacity, and indications of design of other types of engineered risk-reduction struc-previous flows. Deposit parameters include area, surface tures. For deflection walls and berms or for foundationgradient, thickness, gradation, and largest clast size. reinforcement, fan gradient, flow type (debris versusFlow parameters are difficult to determine unless meas- hyperconcentrated versus stream), flow depth, peak flow,ured immediately after an event, and are often inferred flow velocity, and debris size and gradation are importantfrom deposit characteristics or evidence from the feeder to ensure that the structure has the appropriate height,channel. The flow parameters include estimates of flow side slope, and curvature to account for run-up andtype(s), volume, frequency, depth, velocity, peak dis- impact forces. For design of debris barriers, flow vol-charge, and runout distance. ume, depth, deposition area, and gradient are needed to Debris flows can have significantly higher peak dis- determine the appropriate storage volume. The size andcharge than stream-flow flooding. Estimation of peak gradation of debris, and the anticipated flow type aredischarge is critical because it controls maximum veloci- important in the design of debris-straining structures.ty and flow depth, impact forces, ability to overrun pro- Flow types are important to help estimate associatedtective barriers, and runout distance (Hungr, 2000). Van- water volumes. Baldwin and others (1987), VanDineDine (1996) states that debris-flow discharges can be up (1996), Deng (1997), and VanDine and others (1997)to 40 times greater than a 200-year flood, which shows describe other design considerations for debris-flow-con-the importance of carefully estimating peak discharge trol structures.when designing protective structures. Pierson (1985) Even though geologic evaluations use quantitativedescribes flow composition and dynamics of the 1983 and objective procedures, estimating design parametersRudd Canyon debris flow in Davis County, and includes for risk-reduction structures has practical limits. As stat-some flow properties typically considered in engineering ed earlier, historical records of debris flows show flows todesign. Costa (1984) also lists specific physical proper- be highly variable in terms of size, material properties,ties of debris flows. Keaton (1990) describes field and and travel and depositional behavior. Many debris-flow12 Utah Geological Survey

design-parameter estimates have high levels of uncertain- (c) test-pit and trench logs (generallyty and often represent a best approximation of a complex at 1 inch = 5 feet) showingnatural process; therefore, appropriate limitations and descriptions of geologic units,engineering factors of safety must be incorporated in risk- layer thicknesses, maximum grainreduction-structure design. Investigators must clearly sizes, and interpretation of flowstate the limitations of the evaluation methods employed types;and the uncertainties associated with design-parameterestimates. (d) basis for design flow-volume esti- mates (deposit thickness and area estimates); a range of estimates is REPORT GUIDELINES suggested based on maximum, average, and minimum thickness These guidelines supplement the Guidelines for Pre- and area estimates;paring Engineering Geologic Reports in Utah (Associa-tion of Engineering Geologists, Utah Section, 1986) and (e) runout distance, spatial extent,Guidelines for Evaluating Landslide Hazards in Utah thickness, flow type, and deposit(Hylland, 1996) that provide recommendations for engi- characteristics of historical flows,neering geology and landslide reports. The scope of if present;study and techniques used to evaluate debris-flow haz-ards vary depending on the development proposed and (f) deposit age estimates or other evi-site characteristics. Pertinent data, analysis, conclusions, dence used to estimate the fre-and recommendations must be documented in a written quency of past debris flows; andreport. The report must present sufficient information toallow technical reviewers to evaluate the conclusions and (g) an evaluation of the debris-flowrecommendations. The following list summarizes essen- hazard based on anticipated prob-tial report information. ability of occurrence and volume, flow type, flow depth, deposition 1. The scope of the project and intended land use. area, runout, gradation of debris, flow impact forces, and stream- 2. Reference materials used for evaluation (aer- flow inundation and sediment bur- ial photographs, maps, and published and un- ial depths. published reports), including scale and publi- cation date, where appropriate. 6. The drainage basin and channel evaluation should include: 3. A location map (such as part of a 1:24,000- scale U.S. Geological Survey topographic (a) vicinity (1:24,000 scale) geologic quadrangle map) showing the site relative to map on a topographic base of the surrounding physical features and the drain- drainage basin showing bedrock age basin(s) for the alluvial fan(s) at the site. and surficial geology, including shallow landslides (debris slides) 4. One or more site maps at a scale suitable for and a measurement of drainage- site planning (map scale depends on site basin morphologic parameters; and/or development size; recommended site map scale 1 inch = 100 feet) showing pro- (b) an estimate of the susceptibility of posed development (if known), and topogra- the drainage basin to shallow phy at an appropriate contour interval. landsliding, likely landslide vol- ume(s), and volume of historical landslides, if present; 5. The alluvial-fan evaluation should include: (a) site-scale geologic map showing (c) estimates of the susceptibility of areas of active-fan deposition the drainage basin slopes to ero- (generally Holocene-age alluvial sion; fans) and other surficial deposits, including older debris-flow and (d) a longitudinal channel profile, alluvial-fan deposits and their rel- showing gradients from headwa- ative age; ters to the alluvial fan;

length of channel lined by bed- (c) geologic design parameters for

rock, cross-channel profiles, and debris-flow-control structures, as estimated volume of channel sedi- appropriate; implications of risk- ment available for sediment bulk- reduction measures on adjacent ing including estimated bulking properties, and need for long-term rate(s) in cubic yards per linear maintenance; and foot of channel. (d) the residual risk to development 7. If risk-reduction designs are considered, the after risk-reduction measures are following elements should be included: in place.

(a) For debris storage basins, both As noted in 8b above, the geologic evaluation is often alluvial-fan and channel volume only the first step in the debris-flow-hazard evaluation estimates must be compared to and risk-reduction process. Depending on the risk-reduc- select an appropriate design debris tion techniques considered, subsequent hydrologic, hy- volume. For flows that may initi- draulic, and/or engineering studies may be needed to esti- ate as debris slides, an appropriate mate peak flows and water volumes, route sediment, and debris-slide volume must be design control structures. Geologists, hydrologists, and included. Due to uncertainties in- engineers must work as a team to recommend reasonable, herent in both methods, the vol- appropriate, cost-effective risk-reduction techniques. ume estimates may differ signifi- Geologic evaluations of debris-flow hazards must be cantly. Rationale for the chosen performed by a licensed Utah Professional Geologist. volume estimate must be provided. The report must include the geologists professional stamp and signature. The geologist should be an engi- (b) For debris-flow-deflection struc- neering geologist with at least a B.S. in geology or relat- tures or debris-flow-resistant con- ed field, a minimum of 3 years experience in a responsi- struction (reinforcement of foun- ble position in the field of engineering geology, have dations, flood-proofing), hydraul- experience in debris-flow-hazard evaluation, and must ic modeling of debris-flow dis- meet minimum qualifications as defined in local govern- charge, run-up, and runout and ment ordinances. A registered Professional Engineer calculation of impact forces is rec- must stamp all studies that include engineering analysis ommended. Specific information and design. on flow type(s), deposit distribu- tion and thickness, flow velocity, peak flow, and runout is necessary ACKNOWLEDGMENTS to calibrate models. Numerous improvements to early drafts of these 8. Conclusions regarding the geologic evalua- guidelines resulted from critical reviews by many people. tion of the debris-flow hazard should include: Gary Christenson (UGS Geologic Hazards Program Man- ager) provided technical assistance and critically re- (a) the probability of debris-flow viewed the text. Doug VanDine (VanDine Geological occurrence (if possible), estimates Engineering Limited) and Robert Pack (Utah State Uni- of debris-flow volume, delin- versity) also provided critical reviews that greatly eation of hazard areas, and the improved the manuscript. The UGS Geologic Hazards likely effects of debris flows on Program staff provided helpful review comments. Sue the proposed development; Cannon, Tom Pierson, and Gerald Wieczorek (U.S. Geo- logical Survey) provided helpful comments and sugges- (b) recommendations for hydrologic, tions, as did Dale Deiter, Jeff Keaton, Jerry Higgins, hydraulic, and engineering studies Jason Hinkle, Matt Lindon, Dave Noe, Bob Rasely and to define buildable and non-build- Paul Santi. Members of the Association of Engineering able areas (if appropriate) and Geologists Intermountain Section also provided review design risk-reduction measures; comments.14 Utah Geological Survey